Store information about focus reason in QQuickWindowPrivate

QtQuickControl's Desktop style needs to know whether the last focus change
was requested by keyboard or not. With this information the Desktop style
is able to set the QStyleOption accordingly. Without the flag, some of
the QStyles (QFusionStyle, QGtkStyle) don't render a focus rectangle.

This patch is needed by #change,84389 in QtQuickControls

Change-Id: Ia06f56b018cd35eea933892a4c50e0aa7328042d
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Liang Qi <liang.qi@digia.com>